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You don't want to exceed the critical temperature, but you also want the condensing temperature low enough to keep your compression ratio down but enough pressure drop across the metering device to properly feed refrigerant.

This all relies on a lot of different parameters and most systems are different. If you are looking for a rule of thumb, I am not aware of one for this situation.

my question:
how much recommended the refrigerant critical temperature to be higher than the condensing temperature.

I think your question really is:

how far below the refrigerant critical temperature should the condensing temperature be?

The basic answer is: as far as possible.

However, this is more a refrigeration system deign question than a pressures question. If you convert the critical R410a you will see that it is not all that low. My PT chart goes to 613 psi and says that pressure is equal to about 150 degrees. So the only way to.exceed that is to that operate your system with a highrr than 150 degree condenser, which is not likely. Instead you are going to stick to standard design and that by itself keeps your design within range.

The Critical Point is a limiting factor; yes - but it's not much of a useful reference point in relation to 'proper' head pressure.

The lowest head pressure which will provide adequate refrigerant flow at the metering device will always provide the lowest compression ratio across the compressor and so the greatest operating efficiency at the lowest operating cost.

But when the system is being designed Cost will almost always be an important consideration. Larger condensers are better - but they cost more. So while, yes; you are trying to get the operating head pressure as low as possible - you still have to sell the product. Design will always be a compromise between Performance, Cost, and Quality.

You say: "i am studying the HVAC system and trying to completely visualize the working condition for refrigerant throughout all of the process."

This is an excellent approach for you to take and one which is sometimes neglected by students. So please describe what you already know about the Refrigeration Process and let's explore it one area at a time. Everyone here can pitch in and we'll go through it all with you.

PHM
---------------

Originally Posted by mohad_i

hi friends.

i am studying the HVAC system and trying to completely visualize the working condition for refrigerant throw out all process

in the condenser coil the gas will condense at the designed condensing temperature. (which normally can be between 15C higher than the ambient temperature)

my question:
how much recommended the refrigerant critical temperature to be higher than the condensing temperature.

PHM
--------
The conventional view serves to protect us from the painful job of thinking.

thanks you all for the perfect contribution you add,

The Critical Point is a limiting factor; yes - but it's not much of a useful reference point in relation to 'proper' head pressure.

The lowest head pressure which will provide adequate refrigerant flow at the metering device will always provide the lowest compression ratio across the compressor and so the greatest operating efficiency at the lowest operating cost.

But when the system is being designed Cost will almost always be an important consideration. Larger condensers are better - but they cost more. So while, yes; you are trying to get the operating head pressure as low as possible - you still have to sell the product. Design will always be a compromise between Performance, Cost, and Quality.

You say: "i am studying the HVAC system and trying to completely visualize the working condition for refrigerant throughout all of the process."

This is an excellent approach for you to take and one which is sometimes neglected by students. So please describe what you already know about the Refrigeration Process and let's explore it one area at a time. Everyone here can pitch in and we'll go through it all with you.

PHM
---------------

thanks you all for your perfect contribution you add, its all of greet help to me.

what i had in mind for condensing temperature is how much higher should i keep it from the ambient temperature (the condenser split to be high enough to make appropriate delta T and allow for efficient heat transfer) BUT also how far bellow the critical temperature the condensing temperature should be.

as a role of thumb its recommended to keep around 15C between the ambient and condensing temperature, but designing at ambient 50C mean recommended condensing temperature can be around 65C... which makes the condensing temperature only around 5C bellow the critical temperature for R410a..! is this fine?

and being that much close to the critical temperature make the expansion device working on 50/50 vapor/liquid (so most of the refrigerant already vaporized even before the evaporation process with the evaporator coil start... huge loose in efficiency.. what you think?

increasing the condenser coil will lower the condensing temperature but is it the only solution on hand ?

as a role of thumb its recommended to keep around 15C between the ambient and condensing temperature, but designing at ambient 50C mean recommended condensing temperature can be around 65C... which makes the condensing temperature only around 5C bellow the critical temperature for R410a..! is this fine?

First off good for you for asking for help! I get the impression you are taking engineering courses and you are using Google translate. That means to me that you are putting a lot of effort into this interaction. I speak fluent Russian, but would have a hard time discussing refrigeration in that language. So I have sympathy for you.

When I work on refrigeration systems, generally if the liquid line leaving the condenser is more than 3-5 degrees F above ambient air temperature, I am looking at the condenser coil as being dirty. So you may wish to revisit the 15 degrees C rule of thumb. I think it is 1.5 degrees C. That alone may eliminate the issues here. I have seen PHM post before, so I'm sure he will have something better to add.